Editing Gravity Probe A (section)

==Experimental setup==
The {{val|60|u=kg}} Gravity Probe A spacecraft housed an atomic [[hydrogen maser]] system. Maser is an acronym for microwave amplification by stimulated emission of radiation, and is similar to a laser, as it produces coherent [[electromagnetic waves]] in the microwave region of the electromagnetic spectrum. A hydrogen maser produces a very accurate signal (1.42 billion cycles per second), which is highly stable{{mdash}}to one part in a quadrillion ({{10^|15}}). This is equivalent to a clock that drifts by less than two seconds every 100 million years.<ref>{{cite web|url=http://einstein.stanford.edu/content/faqs/gpa1.html|title=Space Probe to Test Einstein's "Space-Time Warp" Theory|last=Milliner|first=Joyce B.|date=June 10, 1976|access-date=May 5, 2013|archive-url=https://web.archive.org/web/20130515182441/http://einstein.stanford.edu/content/faqs/gpa1.html|archive-date=May 15, 2013|url-status=dead}}</ref> A microwave signal derived from the maser frequency was transmitted to the ground throughout the mission. The one-way signal received from the rocket was [[relativistic Doppler shift|relativistically Doppler shifted]] due to the speed of the rocket and in addition was gravitationally Doppler blue-shifted by a minute amount.

In addition to the hydrogen maser carried by the rocket, another hydrogen maser on the ground was used as a source for continuous transmission of a microwave signal to the rocket. A microwave transponder carried on the rocket returned the signal to the Earth. On the way up, the signal received by the rocket was Doppler shifted due to the speed of the rocket and was gravitationally red-shifted by a minute amount. The transponder signal received on the ground was Doppler shifted due to the speed of the rocket and was gravitationally blue-shifted by the same amount that it was red-shifted on the way up. Since the gravitational Doppler shift of the signals on the way up always exactly cancelled the gravitational Doppler shift on its way down, the two-way Doppler shift of the signal received on the ground depended only on the speed of the rocket.     

In a microwave [[frequency mixer]], one-half of the two-way Doppler shift from the transponded ground maser signal was subtracted from the Doppler shift of the space maser. In this way, the Doppler shift due to the spacecraft's motion was completely cancelled out, leaving only the gravitational component of the Doppler shift. 

The probe was launched nearly vertically upward to cause a large change in the gravitational potential, reaching a height of {{cvt|10,000|km|mi}}. At this height, general relativity predicted a clock should run 4.5 parts in {{10^|10}} faster than one on the Earth, or about one second every 73 years.<ref name=posSci>{{cite magazine |last=Gilmore |first=C.P |date=December 1979 |title=After 63 years, why are they still testing Einstein? |url=https://books.google.com/books?id=PAEAAAAAMBAJ&pg=RA6-PA12 |magazine=Popular Science |publisher=Bonnier Corporation |volume=215 |page=12 |issn=0161-7370 |access-date=13 May 2020}}</ref> The maser oscillations represented the ticks of a clock, and by measuring the frequency of the maser as it changed elevation, the effects of gravitational time dilation were detected.